Carbon nanotube-containing catalysts, methods of making, and reactions catalyzed over nanotube catalysts

ABSTRACT

Methods have been developed to form catalysts having active metals disposed on a carbon nanotube coated porous substrate. Catalysts and reactions over nanotube-containing catalysts are also disclosed. Results are presented showing enhanced performance resulting from use of the inventive catalyst. Mesoporous oxide layers can be utilized to improve catalyst properties.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/758,621, which was a continuation of U.S. patent application Ser. No.10/032,207, filed Dec. 21, 2001, now U.S. Pat. No. 6,713,519.

FIELD OF THE INVENTION

The present invention relates to catalysts containing carbon nanotubes,methods of making catalysts containing carbon nanotubes on poroussubstrates, systems employing carbon-nanotube-containing catalysts, andreactions catalyzed in porous carbon nanotube-containing catalysts.

INTRODUCTION

Catalysts are crucially important in controlling chemical reactions invirtually all aspects of our lives. For example, catalysts are used tolower the temperature, increase the rate, and control the products inchemical reactions. While catalysts can be liquids or gases, solidcatalysts are especially attractive for commercial applications becausethey are easy to store and transport, are readily separated from productstreams, tend to be more environmentally benign, and can providesuperior performance and greater control of a reaction.

A well-known problem with solid catalysts is slow heat and/or masstransfer. That is, with solid catalysts and systems employing solidcatalysts, the speed of a chemical reaction can be limited by the timenecessary for heat to travel to or from the catalyst or by the timeneeded for chemicals to get to and from the catalyst.

For many years, scientists and engineers have sought better catalystmaterials with improved heat and/or mass transport properties. Whilethere are probably thousands of publications and patents that addressthese problems, two recent patents are discussed herein.

In one approach, van Wingerden et al., in U.S. Pat. No. 6,099,965,described methods of making catalysts having desirable heat transportproperties. In one example, particles of an iron-chromium alloy areplaced in a steel pipe and sintered in a hydrogen atmosphere. Thesintered particles are then heat treated in air and then treated with asuspension of alumina and Fe₂O₃/Cr₂O₃.

Tennet et al., in U.S. Pat. No. 6,099,965, described a catalystcomprising a rigid carbon nanotube structure and a catalyticallyeffective amount of a catalyst supported thereon. Numerous advantages ofthis structure including enhanced heat and mass transfer are dicussed(see col. 16, lines 8-65).

Despite the prior work, there remains a need for novel solid catalystmaterials that have superior heat and/or mass transport capabilities.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides an engineered catalyst thatincludes a support material having through-porosity (defined asdiscussed below), a layer comprising carbon nanotubes on the supportmaterial; and a surface-exposed catalytically-active composition.

In another aspect, the invention provides catalyst including a support;nanotubes dispersed over the support; and a catalytically-activecomposition dispersed over the nanotubes.

In yet another aspect, the invention provides a method of forming aporous carbon nanotube containing catalyst structure. In this method, alarge pore support is provided having through porosity. Carbon nanotubesare formed over the large pore support, and a catalyst composition isdeposited over the carbon nanotubes.

The invention also includes methods of conducting catalyzing chemicalreactions in which one or more reactants are contacted with any of thecarbon nanotube containing catalysts described herein. In this method,the one or more reactants react to form a product. The catalystcatalyzes the reaction relative the same reaction conducted in theabsence of a catalyst. For example, the invention provides aFischer-Tropsch process in which a gaseous composition, comprising COand hydrogen, is passed over any of the carbon nanotube containingcatalysts described herein.

The invention also provides a catalytic process for aqueous phasehydrogenations to produce higher value chemical products from biomassfeedstock.

In another aspect, the invention provides a process of making a porous,carbon nanotube-containing structure, comprising: providing a supportmaterial having through-porosity; depositing seed particles on thesupport material to form a seeded support material; and heating thesupport material and simultaneously exposing the seeded support to acarbon nanotube precursor gas to grow carbon nanotubes on the surface ofthe seeded support material.

In another aspect, the invention provides a porouscarbon-nanotube-containing structure that includes a large pore supporthaving through porosity; and carbon nanotubes disposed over the largepore support.

In still another aspect, the invention provides a method of making acarbon-nanotube-containing structure in which a surfactant templatecomposition (a composition containing a surfactant and silica or silicaprecursors) is applied onto a support. Carbon nanotubes are then grownover the layer made from the surfactant template composition.

The invention also provides processes of using carbonnanotube-containing structures. Preferably, any of the carbonnanotube-containing structures described herein can be used in processesincluding: adsorption, ion exchange, separation of chemical components,filtration, storage of gases (for example, hydrogen or carbon dioxide),distillation (including reactive distillation), as a support structurefor chemical or biological sensors, and as a component in a heatexchanger. Features of carbon nanotube-containing structures that makethese structures particularly advantageous include: high surface area,excellent thermal conductivity, capillary force for enhancedcondensation, and good attractive force for certain organic species.

Thus, the invention provides a method of adsorbing a chemical componentin which a chemical component is contacted with a carbonnanotube-containing structure and the chemical component is adsorbed onthe surface of the carbon nanotube-containing structure. A preferredchemical species is hydrogen. In a preferred embodiment, the exteriorsurface of the carbon nanotube-containing structure is a palladiumcoating. In preferred embodiments, the adsorption is run reversibly in aprocess such as pressure swing or temperature swing adsorption. Thismethod is not limited to adsorbing a single component but includessimultaneous adsorption of numerous components.

Similarly, the invention provides a method of separating a chemicalcomponent from a mixture of components. “Mixture” also includessolutions, and “separating” means changing the concentration of at leastone component relative to the concentration of at least one othercomponent in the mixture and preferably changes the concentration of atleast one component by at least 50% (more preferably at least 95%)relative to at least one other component—for example reducing theconcentration of a 2 M feed stream to 1 M or less. Particular types ofseparations include filtration, selective adsorption, distillation andion exchange. Filtering can be accomplished, for example, by passing amixture having at least two phases through a porous carbonnanotube-containing structure where at least one of the phases getsphysically caught in the structure. A carbon nanotube-containingstructure with surface-exposed carbon nanotubes can function efficientlyfor the separation of some organics because the nanotubes can behydrophobic while organics can be adsorbed quite well. For ion exchangeit is desirable to coat the surface with an ion exchange agent.

The preparation of porous materials, such as foams, coated with carbonnanotubes and a high-surface area metal oxide coating, can be difficult.The locally aligned nanotubes exhibit high surface Van der Waal forcesand hydrophobic properties. Conventional wash coating of metal oxidesusing aqueous based solution often results in a non-uniform coating orpoor adhesion onto the nanotubes. We have developed treatment methods tomodify the surface properties of the nanotubes, making this new class ofmaterials possible for application as engineered catalyst structure. Wehave fabricated carbon nanotube-based engineered catalyst and havedemonstrated its performance for Fisher-Tropsch reaction in amicrochannel reactor. Under operating conditions typical of microchannelreactors with minimal heat and mass transfer limitations, it was foundthat the integrated nanotubes substrate has further improved theperformance, as indicated by enhanced reaction rate and improved productselectivity. This concept can also be applied toward conventionalreactors, which operate under severe heat and mass transfer inhibitionswith catalyst performance far less than that predicted from theintrinsic kinetics.

Various embodiments of the present invention can offer numerousadvantages, including: creating larger pores through whichreactants/products transport to the catalytic sites, improved heattransport, controlling the direction of heat transport, enhanced surfacearea, excellent thermal stability, excellent thermal conductivity,reduced mass transfer limitations, utility in microreactors, readyadaptability in fixed-bed type reactors, and increased catalyst loadinglevels.

The surface area enhancement that arises from these nanoscale fibers cangreatly increase the catalyst site density within a fixed reactorvolume. The potential to create larger pore size naturally generatedfrom the interstices between carbon nanotubes can be beneficial forreactions involving both gas and liquid phases liquid reactants orproducts on a solid catalyst, since the transport of gas phase moleculesthrough the liquid phase inside the pores is often the rate-limitingstep which not only hinders the reaction rate but also adversely affectsproduct selectivity.

In this application, “pore size” and “pore size distribution” can havedifferent meanings as explained below. “Pore size” can be measured by(optical or electron) microscopy where pore size distribution and porevolume are determined statistically from counting in a field of view (ofa representative portion of the material) and pore size of each pore isthe average pore diameter. Pore size is determined by plotting porevolume (for large pore materials the volume of pores having a size ofless than 100 nm can ignored) vs. pore size and “average pore size” isthe pore size at 50% of the existing pore volume (e.g., for a materialthat has a 40% pore volume, the “average pore size” is the size of thelargest sized pore that adds with all smaller sized pores to reach 20%pore volume). Where practicable, the pore size and pore volume aremeasured on a cross-section of the material that may be obtained with adiamond bladed saw. For an isotropic material any representativecross-section should produce the same results. For anisotropic materialsthe cross-section is cut perpendicular to maximum pore length.

Alternatively, pore size and pore size distribution can be measured bynitrogen adsorption and mercury porisimetry.

A “large pore” support (or other material) is a support that ischaracterized by the presence of pores having a pore size (diameter) ofat least 100 nm, more preferably at least 1 μm, and in some embodiments500 nm to 400 μm. Preferably, these supports have through porosity, suchas in honeycombs, foams or felts.

“Through porosity” means that (1) when a “through porosity” material issized (sized means cut or grown—that is, a through porosity materialneed not be 1 cm in length, but for testing purposes could be grown ormanufactured) to a length of 1 cm (or at least 0.1 cm if 1 cm isunavailable) and oriented in the direction of maximum flow, a measurableamount of argon gas will flow through the intact material, and (2) across-section taken at any point perpendicular to flow (for example,where the material is disposed within a reactor) shows the presence ofpores, and, in the large pore materials, the presence of large pores. Inthe present invention, the interstices between packed, unsintered powderparticles or pellets do not qualify as through porosity (althoughpowders sintered to form larger materials would qualify). By definition,materials having only pitted surfaces (such as anodized aluminum) do nothave through porosity, and mesoporous silica (by itself) does not havethrough porosity. Anodized aluminum is not a through porosity material.

A “carbon nanotube” is primarily or completely carbon in a substantiallycylindrical or rod-like form having a diameter of less than 200 nm,preferably in the range of 4 to 100 nm. “Nanotubes” may include bothtubes and rods.

An “engineered catalyst” means a catalyst having a porous support,carbon nanotubes, and a catalytically active material disposed over atleast a portion of the nanotubes.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 a is a scanning electron micrograph (SEM) of a metal alloy foam.

FIG. 1 b shows an SEM of a metal foam having a coating of carbonnanotubes.

FIG. 1 c is a higher magnification view of the foam of FIG. 1 b.

FIG. 1 d is a higher magnification view of the foam of FIG. 1 b.

FIG. 2 a shows an SEM of a foam having a coating of carbon nanotubes.

FIG. 2 b shows an SEM of a foam having a coating of carbon nanotubesafter a longer exposure to conditions for nanotube growth.

FIG. 2 c shows an SEM of a foam having a coating of carbon nanotubesafter a still longer exposure to conditions for nanotube growth.

FIG. 3 a shows an SEM of a ceramic foam having a coating of carbonnanotubes.

FIG. 3 b is a higher magnification view of the foam of FIG. 3 a.

FIG. 4 a shows an SEM of a metal foam having a coating of carbonnanotubes and a surface wash coat of alumina.

FIG. 4 b is a higher magnification view of the foam of FIG. 4 a.

FIG. 4 c is a higher magnification view of the foam of FIG. 4 a.

FIG. 5 is an illustration of a microreactor 10, including reactionchamber 18 in which reactants are converted to products 24. An optionalthermally conductive separation plate could be used to separate thereaction chamber from microchannel heat exchanger 12, through whichflows heat exchange fluid 26.

DETAILED DESCRIPTION OF THE INVENTION

In some embodiments, support materials of the present inventionpreferably have through-porosity, preferably these pathways aresufficiently large to allow molecular diffusion at room temperature. Insome preferred embodiments, the support is a porous material having apore volume of 5 to 98%, more preferably 30 to 95% of the total porousmaterial's volume. Preferably, at least 20% (more preferably at least50%) of the material's pore volume is composed of pores in the size(diameter) range of 0.1 to 300 microns, more preferably 0.3 to 200microns, and still more preferably 1 to 100 microns. Pore volume andpore size distribution are measured by Mercury porisimetry (assumingcylindrical geometry of the pores) and nitrogen adsorption. As is known,mercury porisimetry and nitrogen adsorption are complementary techniqueswith mercury porisimetry being more accurate for measuring large poresizes (larger than 30 nm) and nitrogen adsorption more accurate forsmall pores (less than 50 nm). Pore sizes in the range of about 0.1 to300 microns enable molecules to diffuse molecularly through thematerials under most gas phase catalysis conditions. Preferred forms ofthe support are foams, felts (i.e., nonwoven accumulations of strands orfibers), meshes, membranes, and honeycombs. In some particularlypreferred embodiments, the support has tortuous porosity withinterconnected passages, such as in foams; this tortuosity creates moredesirable flow patterns for better mixing and heat transfer. Anotherpreferred form is a microchannel (that is, a channel having a widthand/or height of 1 mm or less) array. Other embodiments of the inventioncan have supports of larger dimensions, for example a minimum dimensionof 1 cm (i.e., each of height, length and width are more than 1 cm).

The support can be made of a variety of materials such as ceramic, butin embodiments requiring rapid heat transport, the support preferably isa thermally conductive material such as a metal. In some particularlypreferred embodiments, the support is stainless steel, or an alloy suchas monel. In other embodiments, preferred support materials includecordierite, silica, alumina, rutile, mullite, zirconia, silicon carbide,aluminosilicate, stabilized zironia, steel and alumina-zirconia blend.For use in hydrothermal conditions, preferred supports include zirconiaand carbon.

In other embodiments, where through porosity is not necessary, thesupport can be a thin membrane of anodized aluminum (a macroporousaluminum oxide membrane) or other macroporous membrane (with a volumeaverage pore diameter of at least 20 nm, for example, commerciallyavailable ceramic membranes with the macropores straight through themembrane thickness) that is optionally treated with a surfactanttemplate composition such that mesoporous silica substantially fills themacropores. Carbon nanotubes are applied to form a membrane that can beused, for example, as a molecular sieve, an adsorbent, or treated withan ion exchange medium. The inventive structures made with an adherentmesoporous silica layer disposed between the support and the carbonnanotubes may or may not have through porosity. “Substantially fills”the macropores means that the mesoporous silica extends completelyacross the diameter of a macropore but does not necessarily completelyfill each macropore.

The carbon nanotubes are preferably at least 90 mol % C, more preferablyat least 99 mol % C. The nanotubes may have a metallic nanoparticle(typically Fe) at the tips of the nanotubes. The nanotubes have a lengthto width aspect ratio of at least 3; more preferably at least 10. Thenanotubes preferably have a length of at least 1 μm, more preferably 5to 200 m; and preferably have a width of 3 to 100 nm. In some preferredembodiments, as measured by SEM, at least 50% of the nanotubes have alength of 10 to 100 μm. Preferably, of the total carbon, as measured bySEM or TEM, at least 50%, more preferably, at least 80%, and still morepreferably, at least 90% of the carbon is in nanotube form as comparedto amorphous or simple graphite form.

Depending on the intended use, the distribution of nanotubes can betailored to obtain the desired characteristics, for example, surfacearea and thermal transport. The nanotubes preferably have an averageseparation (from central axis to central axis, as measured by SEM) offrom 10 to 200 nm, more preferably 20 to 100 nm. Having close neighbors,means that the nanotubes will be highly aligned. In some preferredembodiments, the nanotubes are sufficiently dense to cover theunderlying support, as measured by SEM. In some preferred embodiments,the material includes nanotubes arranged in clumps on the support wherethere is a high degree of nanotube alignment within each clump (see,e.g., FIG. 1 b). Preferably, the surface area of the article, asmeasured by BET/N₂ adsorption, is at least 50 m²/g nanotubes, in someembodiments 100 to 200 m²/g nanotubes; and/or at least 10 m²/g(nanotubes+support), in some embodiments 10 to 50 m²/g(nanotubes+support). Size and spacing of the carbon nanotubes can becontrolled by control of the surfactant template composition; forexample, larger diameter nanotubes can be obtained by use of largersurfactant molecules.

A “catalyst composition” is a composition of matter that will catalyze achemical reaction. Preferred embodiments of the invention include acatalyst composition that is exposed on at least one surface. Theinvention is not limited to specific catalyst types. In applicationswhere a catalyst composition (or a catalyst composition precursor) isdeposited directly on the nanotubes, the catalyst composition may be anyof the catalysts used on carbon supports. Additional layers can bedeposited on the nanotubes to support a desired catalyst. Typicalcatalysts include metals and metal oxides. Especially preferredcatalysts include: Fischer-Tropsch catalysts (to cite one example,Co-based catalysts), and steam reforming catalysts. Knowledge of thescientific literature and routine experimentation can be used by skilledworkers to select appropriate catalyst compositions for reactions suchas acetylation, addition reactions, alkylation, dealkylation,hydrodealkylation, reductive alkylation, amination, aromatization,arylation, carbonylation, decarbonylation, reductive carbonylation,carboxylation, reductive carboxylation, reductive coupling,condensation, cracking, hydrocracking, cyclization,cyclooligomerization, dehalogenation, dimerization, epoxidation,esterification, exchange, halogenation, hydrohalogenation, homologation,hydration, dehydration, hydrogenation, dehydrogenation,hydrocarboxylation, hydroformylation, hydrogenolysis, hydrometallation,hydrosilation, hydrolysis, hydrotreating,hydrodesulferization/hydrodenitrogenation (HDS/HDN), isomerization,methanol synthesis, methylation, demethylation, metathesis, nitration,partial oxidation, polymerization, reduction, steam and carbon dioxidereforming, sulfonation, telomerization, transesterification,trimerization, water gas shift (WGS), and reverse water gas shift(RWGS).

The inventive structures may include additional materials such ascarbide, nitride, sulfide, or oxide layers or metal layers. Aparticularly preferred interlayer material is a mesoporous thin silicafilm, preferably disposed between the support and the nanotubes.Mesoporous materials are described in publications such as U.S. Pat. No.5,645,891, which is incorporated herein, and D. Zhao, P. Yang, N.Melosh, J. Feng, B. F. Chmelka, and G. D. Stucky, Adv. Mater., 1998,Vol. 10, No. 16, P1380-1385. These mesoporous interlayers offer numerousadvantages including enhanced surface area (typically 800 to 1000 m²/gsilica) and providing uniform microenvironments for catalysis (forexample, while the supports can be anisotropic, the mesoporousinterlayer can put the same material and same pore size throughout thestructure—this promotes uniform deposition of nanoparticle seeds and,therefore, uniform nanotubes). In some preferred embodiments, themesoporous film is codeposited with metallic nanoparticles, such as Fenanoparticles, that serve as seeds for nanotube growth.

In another preferred embodiment, an oxide layer or layers are disposedbetween the support and the nanotubes. A thin oxide layer can be formedon metal supports, for example, by heat treatment in the presence ofoxygen. This oxide layer can improve adhesion of subsequent oxidelayers, and protect the underlying support from degradation.Alternatively, or in addition, a metal oxide layer can be deposited overthe support. For example, an alumina layer can be deposited (preferablya dense layer applied by chemical vapor deposition) onto the supportbefore applying the nanotubes. The oxide layer(s) may enhance adhesionbetween the ceramic-metal interface, as well as protecting theunderlying support from degradation during preparation or use. An oxidelayer is especially desirable over a metal support. The thickness of theoxide layer(s) in some embodiments is preferably less than about 200 nm,and, in some embodiments is in the range of 0.05 to 5 μm, and in someembodiments is in the range of 100 to 1000 nm. Ideally, these oxidelayer(s) should be thick enough to promote formation of a dense nanotubelayer, but thin enough to have little adverse affect on thermalconductivity.

An oxide layer can be disposed over the carbon nanotubes by washcoatingor vapor coating. In a preferred embodiment, the nanotubes surface canbe oxidized (or partly oxidized). Oxidation can be done, for example, byexposure to air at elevated temperature. Preferred conditions are 350 to500° C. for at least one minute; more preferably 400 to 500° C. for 2 to50 minutes. Other oxidation methods could alternatively be used, forexample, treatment with an acidic solution, or coating with a layer ofvapor deposited hydrophilic material. It is believed that thesetreatments modify the nanotube surfaces such that the washcoatingsolution is absorbed into the interstices during subsequent coatingsteps.

In especially preferred embodiments, an oxide layer is, or includes, amesoporous silica layer. A mesoporous silica layer may be formed alongwith nanoparticles which seed the growth of carbon nanotubes. Eachmesoporous silica layer preferably has a thickness of between 0.5 and 3μm.

In another embodiment a carbon nanotube-containing includes a layer ofan electroactive compound, preferably an electroactive polymer, disposedeither between the support and the nanotubes or over the nanotubes. Suchstructures can be used to separate chemical components. In theseembodiments, the support is preferably electrically conductive such as ametal mesh.

Some inventive catalysts or carbon nanotube-containing structures can becharacterized by their properties. In this context, catalysts aredefined to include all the components including support, nanotubes andcatalytically active component (such as a reduced metal). The catalystsor carbon nanotube-containing structures preferably have throughporosity, preferably have large pores, and are preferably a porousmaterial having a pore volume of 5 to 98%, more preferably 30 to 95% ofthe total engineered catalyst's or carbon nanotube-containingstructure's volume. Preferably, at least 20% (more preferably at least50%) of the material's pore volume is composed of pores in the size(diameter) range of 0.1 to 300 microns, more preferably 0.3 to 200microns, and still more preferably 1 to 100 microns. The engineeredcatalyst or carbon nanotube-containing structures preferably has asurface area of at least 0.05 m²/g, more preferably at least 0.5 m²/g,and in some embodiments between 0.1 and 100 m²/g. The catalyst or carbonnanotube-containing structure is preferably not a powder, and morepreferably is a monolith having a volume (including voids) of at least 5mm³, and in some embodiments 5 to 5000 mm³. In preferred embodiments, areactor could have monolith packing (that would have many smallmonoliths) or large catalyst inserts, where a single piece is loadedinto the reactor. Alternatively, or carbon nanotube-containing catalystscan be in the form of pellets or powders and used in conventional fixedor fluidized bed reactors.

The inventive catalysts can also be characterized by their reactivity.For example, a Fischer-Tropsch catalyst according to the presentinvention, when tested at 265° C., at 16 atm, a H₂/CO ratio of 2, and a250 msec contact time, preferably exhibits: a CO conversion of at least25%, more preferably at least 35%, and in some embodiments about 30 toabout 45%, a methane selectivity of less than 35%, more preferably lessthan 30%, and in some embodiments about 35 to about 25%; and a specificactivity (defined as mmol CO converted per gram of total metal (whichmay include Co+Re, etc. but does not include metal in oxide support) perhour) of at least 1500, more preferably at least 2000, and in someembodiments 1800 to about 2400.

In some preferred embodiments, the fabricated catalyst or carbonnanotube-containing structures contains 0.1 to 20 weight % carbon.

The inventive structures are preferably disposed within microdevicessuch as microreactors with integral or adjacent heat exchangers,preferably microchannel heat exchangers. Examples of reactorconfigurations are disclosed in U.S. Pat. No. 6,680,044, which isincorporated herein by reference as if reproduced in full below. In onepreferred embodiment, the invention comprises a reaction chamber and atleast one adjacent heat exchange chamber. The catalyst (includingsupport, nanotube layer and catalyst) can be sized to match the flowpath of the reaction chamber such that flow is substantially through thepores of, rather than around the body of, the catalyst. In somepreferred embodiments, the engineered catalyst (including voids withinthe catalyst) occupies at least 80%, more preferably at least 95%, of across-sectional area of the interior of a reactor chamber. Preferably,the engineered catalyst is a single piece (monolith) or line of piecesin the reaction chamber occupying at least 80 or 95% of thecross-sectional area of the interior of a reactor chamber. Preferably,the engineered catalyst is a removable piece or pieces rather than acoating.

Other devices for alternative embodiments of the invention includedevices for distillation (such as described in U.S. Pat. No. 6,875,247by TeGrotenhuis et al. filed Dec. 5, 2001 which is incorporated hereinby reference as if reproduced in full below) including reactivedistillation, gas storage (such as devices for swing adsorptiondescribed in U.S. patent application Ser. No. 09/845,777 filed Apr. 30,2001 which is incorporated herein by reference as if reproduced in fullbelow)

Support materials can be obtained commercially. Supports can also bemade by known techniques. Optionally, an intermediate layer or layerscan be applied to the support. The intermediate layer(s) can be appliedby known methods such as wash coating and vapor deposition. The support,or intermediate layer if present, is then seeded with nanoparticles,preferably iron nanoparticles. This can be achieved by applying anaqueous metal solution followed by calcination to form thenanoparticles.

In some particularly preferred embodiments, a mesoporous silica layer isdeposited on the support. See the Examples for a description of asuitable technique for depositing a mesoporous layer. The mesoporoussilica layer can be formed from compositions containing silicaprecursors and surfactant. To make nanoparticles for seeding nanotubes,the composition may also contain a nanoparticle precursor such as atransition metal complex. In preferred embodiments, a compositioncomprising surfactant and silica precursor, or a composition containingthe transition metal complex, or both, are aged before they are combined(aging can be, for example, at least 5 minutes or at least 30 minutes);this allows the hydrolysis reaction to proceed before combining.Preferably a composition containing surfactant and silica precursor alsoincludes an acid, preferably HCl. Preferably a composition containingsurfactant and silica precursor also includes an alcohol, preferably thesilica precursor is a silicon alkoxide and the alcohol has the samehydrocarbon moiety as the alkoxide. The higher the amount of surfactantand TEOS in the solution, the thicker the resulting coating. Thecomposition for forming a mesoporous layer can be coated, preferably bydip coating or spray coating, and then dried and heated in air,preferably at a temperature of 10 to 500° C. Dip coating typicallyresults in a coating thickness of about 1 μm. The thickness of themesoporous silica layer is preferably at least 1 μm, and in someembodiments 1 to 5 μm. Coatings less than 1 μm result in undesirablysparse nanotubes. Mesoporous coatings thicker than about 5 μm areundesirable because cracks will form during drying which can lead toflaking (nonadherence). One dip coating is preferred over multi-dipsbecause a second coating will seal the pore mouths of the firstlayer—thus the pores of the first layer cannot be effectively used fornanotube growth. Templating agents such as C₁₆EO₁₈ can increase the sizeand spaciong of nanotubes as compared with smaller agents.

Carbon nanotubes may be formed by pyrolysis of a carbon-containing gassuch as ethylene, acetylene or CO. Preferably the nanotubes are grown at600-1000° C., with tube length increasing with time. For higher purity,growth is conducted in alternating cycles of tube growth and oxidationto remove amorphous carbon. If desired, the nanotubes may be treatedsuch as by heating in air to form an oxidized surface. Preferably thesurface is oxidized to a sufficient extent to make the surfacehydrophilic, preferably with a static contact angle of less than 30°.

A catalytically active component or components (typically throughcatalyst composition precursors which are subsequently treated toproduce a catalyst composition) can be applied directly on the nanotubesor over intermediate layer(s) disposed over the nanotubes. The as-growncarbon nanotubes are hydrophobic in nature, thus, aqueous or other polarsolvents containing metal or oxide catalyst precursors absorb minimallyon these nanotube sponges. For this reason, surface treatment prior todip coating is highly desirable to modify the surface properties. Inorder to enhance the absorption of catalyst precursors or othercompositions, the wall of the nanotube sponges may be oxidized, forexample, at moderate temperature in the presence of O₂, etched in anacid solution (preferably nitric acid), or exposed to a peroxide. Afteroxidation, the uptake of the precursor solvents increases dramatically.After dip coating, the substrate is annealed at high temperature toremove the H₂O absorbed by capillary forces within the sponge structureand to decompose the metal precursors. Alternatively to dip coating,catalyst component or catalyst component precursors can be applied bywash coating, vapor depositing, electrolytically depositing ordepositing in nonpolar solvents.

The nanotubes can also be functionalized by treatment with a diene orknown functionalizing reagents.

The nanotube layer and/or a catalyst structure can be treated to obtaina hydrophilic or hydrophobic surface depending on the intended use.

Another approach for depositing a layer of thin oxide film on thenanotube structure is to immerse the substrate into a surfactanttemplated sol solution containing the ceramic and metal precursors. Inthis case, the templated liquid crystals preferentially anchor on thesurface of the nanotubes. Subsequent drying and annealing at hightemperature removes the surfactant molecules, resulting in an oxidelayer with well-defined pore structures that adhere strongly onto thenanotube surface. The physical properties of the oxide formed depend onthe surfactant/alcohol/water/precursor ratio.

Chemical reactions using the carbon-nanotube-containing catalysts arealso part of the invention. Examples of these reactions include:synthesis of hydrocarbons from CO and H₂, steam reforming, acetylation,addition reactions, alkylation, dealkylation, hydrodealkylation,reductive alkylation, amination, aromatization, arylation,carbonylation, decarbonylation, reductive carbonylation, carboxylation,reductive carboxylation, reductive coupling, condensation, cracking,hydrocracking, cyclization, cyclooligomerization, dehalogenation,dimerization, epoxidation, esterification, exchange, halogenation,hydrohalogenation, homologation, hydration, dehydration, hydrogenation,dehydrogenation, hydrocarboxylation, hydroformylation, hydrogenolysis,hydrometallation, hydrosilation, hydrolysis, hydrotreating,hydrodesulferization/hydrodenitrogenation (HDS/HDN), isomerization,methanol synthesis, methylation, demethylation, metathesis, nitration,partial oxidation, polymerization, reduction, steam and carbon dioxidereforming, sulfonation, telomerization, transesterification,trimerization, water gas shift (WGS), and reverse water gas shift(RWGS). Reactions can be in liquid, gas, or liquid/gas, andgas/liquid/solid phase. Particular advantages with the inventivecatalyst structure are expected in the reactions where heat and masstransfer limitations are significant, such as liquid phase oxidation andhydrogenation reactions.

The inventive carbon nanotube-containing structure can also beadvantageously used in aqueous phase and hydrothermal conditions, forexample, as a catalyst support. Examples of aqueous phase reactions andhydrothermal reactions and conditions are described in Elliott et al.,U.S. Pat. Nos. 5,814,112, 5,977,013, 6,152,975 and 6,235,797, which areincorporated herein as if reproduced in full below. Thus, the inventionincludes any of the aqueous phase reactions described in the foregoingpatents in combination with the inventive catalysts.

EXAMPLES

Layered, aligned carbon nanotubes on metal and ceramic foams wereprepared and characterized. The figures show various SEM views ofvarious coated and uncoated foams. FIG. 1 a is an SEM view of a FeCrAlYalloy foam. FIG. 1 b shows the same type of foam after depositing carbonnanotubes. From the photomicrograph, one can see chunks of alignednanotubes coating the foam while leaving large pores through thestructure. Higher magnification views are shown in FIGS. 1 c and 1 d. InFIG. 1 d the carbon nanotubes appear curved and wavy (kinked). Thus, thenanotubes have a local alignment (see, e.g., FIGS. 1 b, 1 c) but ajumbled, high surface area orientation at a very high magnification.Thus showing a preferred arrangement in which nanotube alignment isobserved at 2000× magnification while substantial kinkiness is observedat 20,000×. The interior of a nanotube-coated metal foam support wasviewed by SEM of a cross-sectional cut (not shown) demonstrated that thetechnique was effective to cover surfaces throughout a large poresupport.

The effects of extending the period of nanotube growth conditions can beseen by comparing FIG. 2 a with 2 b (longer growth time) and 2 c(longest growth time). Extensive nanotube growth may have the effect ofclosing up the pore structure of a large pore support. SEM views ofcarbon nanotubes on ceramic monolith at various magnifications are shownin FIGS. 3 a-3 c. The appearance is very similar to the growth on metalfoams under similar conditions.

FIGS. 4 a-4 c show SEM views of an FeCrAlY foam that has been coated (asdescribed below) with an alumina layer, a mesoporous silica layer, alayer of carbon nanotubes, and a surface layer of alumina. The alignmentof carbon nanotubes can be seen if we view nanotubes perpendicular tothe nanotube films. However, if nanotubes are viewed from the top of thenanotube films, they appear random as shown in FIG. 4 b.

Preparation of Carbon Nanotube Coated Metal Foam

FeCrAlY intermetallic alloy foam (80 ppi, 85% porosity, purchased fromPorvair, Hendersonville, N.C.) was heat treated by exposing to air at atemperature of at least 800° C. (ramping rate of 20° C./min, 2 htreatment). The heat treatment results in the formation of a layer ofalpha alumina on the surface due to either outward diffusion of Al orinward diffusion of oxygen. Fe/mesoporous silica was coated onto thefoam, with a thickness preferably ranging from 0.1 μm to 5 μm, using adip-coating method. This Fe/mesoporous silica was prepared from atemplating surfactant composition containing a mixture of alcohol,surfactant, water, silica, and iron precursors. To enhance the adhesionbetween the metal foam and the Fe/mesoporous silica catalyst, a denseand pinhole-free interfacial layer was first coated onto the oxidizedFeCrAlY foam by metal organic chemical vapor deposition (MOCVD). Thisinterfacial layer can be Al₂O₃, Al₂O₃+SiO₂, or TiO₂, etc. For example,when TiO₂ was coated, titanium isopropoxide (Strem Chemical,Newburyport, Mass.) was vapor deposited at a temperature ranging from250 to 900° C. at a pressure of 0.1 to 100 torr. Titania coatings withexcellent adhesion to the foam were obtained at a deposition temperatureof 600° C. and a reactor pressure of 3 torr. This layer not onlyincreases the adhesion between metal foam and the Fe/mesoporous silica,it also protects the FeCrAlY from corrosion during the subsequent carbonnanotube growth and reaction.

A layer of Fe/mesoporous silica was dip-coated onto the foam from atemplating surfactant composition to yield a high surface area silicalayer containing dispersed Fe particles. High surface area ensures ahigh carbon nanotube growth rate and a high carbon nanotube density persurface area. Upon high temperature calcination, the precursorsdecomposed to form a layer of Fe/SiO₂, typically with a thickness of0.5-3 μm, which served as the seeding layer for carbon nanotube growth.It should be noted that preparation of this seeding layer is not limitedto the one step dip-coating method with a Fe containing templatingsurfactant composition, as mentioned above. It can also be done byconventional methods such as incipient wetness impregnation or vaporphase impregnation of Fe precursors on a pre-deposited silica layer. Inaddition, the choice of catalytic active metals is not limited to Fe,other metals such as Co and Ni have been demonstrated to be able todecompose gas phase carbon containing molecules to form carbonnanotubes.

As compared to using a wash coat slurry formed from preformed Fe/SiO₂powder onto the metal foam, dip coating the foam substrate with a solgel solution has unexpectedly been found to have significant advantages.The dip coating approach uses direct gelation method, where the solsolution is gelled and dried onto the metal foam. During the drying andgelation process, the mesoporous materials adhere strongly onto themetal foam, thus providing an intimate contact between the foam and theFe/SiO₂ coating without any void micropores on the interface which wouldhamper heat transfer in the final engineered catalyst during reaction.

The carbon nanotube growth was carried out at temperatures of 600-1000°C., depending on the carbon sources and the catalysts. A carboncontaining gas source such as ethylene, acetylene, CO, was introduced tothe substrate for 5-20 min, where the well-aligned carbon nanotubes wereformed by reaction between the C gas source and the Fe particles. Inthis particular example, ethylene was used. The growth rate and lengthof carbon nanotubes were controlled by both temperature and duration.During the growth, the gas was introduced into the chamber at a periodof not more than 20 min, since longer growth time resulted in depositionof amorphous carbon and randomly aligned tubes. If necessary, a growthcycle was conducted. Between the growths, O₂/N₂ with <500 ppm of O₂ wasintroduced to the chamber for 5 min to oxidize any amorphous carbondeposited. A growth rate of 0.5-5 μm/min was typically observed.

The as grown carbon nanotubes are hydrophobic in nature, thus,conventional aqueous solutions containing metal or metal oxide catalystprecursors adsorb minimally on these nanotubes sponges. For this reason,surface treatment prior to dip coating of catalytic components is highlydesirable to modify surface properties. In order to enhance theadsorption of catalyst precursors, the nanotube sponges can be eitheroxidized at moderate temperature (such as 450° C.) in the presence of O₂for 5 min or etching in a nitric acid solution (at room temperature).After treated with these procedures, the uptake of the precursorsolvents increases dramatically. After dip coating, the substrate isannealed at high temperature to remove the H₂O absorbed by capillaryforces within the sponge structure and to decompose the metalprecursors.

In order to further modify the surface of carbon nanotubes, a layer ofthin oxide film can be deposited on the treated carbon nanotubes byimmersing the substrate into a surfactant templated sol solutioncontaining the ceramic and metal precursors. In this case, templatedliquid crystals preferentially anchor on the surface of the nanotubes.Subsequent drying and annealing at high temperature removes thesurfactant molecules, resulting in an oxide layer with well-defined porestructures that adhere strongly onto the nanotube surface. The physicalproperties of the oxide formed depend on thesurfactant/alcohol/water/precursor ratio.

In an alternate method, nano-size Al₂O₃ particles were deposited bydipping carbon nanotube coated metal foams in an Al₂O₃ colloid solution.The weight percentage of the colloid solution varied from 5% to 20% tocontrol the Al₂O₃ loading in the carbon nanotube sponge. The solvent ofthe colloid solution also varied from pure water to 25 wt. % water/75wt. % ethanol to control the surface tension of the colloid solution.After dipping in the colloid solution for >1 min, the coated metal foamswere removed from the solution, and the excess solution was then removedon filter papers. The coated metal foams were rapidly dried in a fewminutes under vacuum (<1 torr) at room temperature. Low temperatureannealing (e.g. 450° C. in air for 0.5 hr) was necessary to completelyremove the solvent. FIGS. 3 a and 3 b show typical SEM images of analumina-coated-carbon-nanotube-coated-metal-foam that was coated using 8wt. % Al₂O₃ colloid solution.

Preparation of Fischer-Tropsch Catalysts

Using the above method, an engineered catalyst was fabricated. TheFeCrAlY intermetallic foam (80 ppi, 85% porosity) with the dimensions of0.30″×1.4″×0.06″ was first oxidized at 900° C. in air for 2 h, and thencoated with a submicron layer of Al₂O₃ using MOCVD. The MOCVD wascarried out using aluminum isopropoxide as the precursor with N₂ carriergas in an oxidizing environment containing 14 vol % of O₂ under 5 Torrat 850° C. The aluminum isopropoxide precursor was stored in a bubblerwhere the vapor pressure was controlled by changing the bubblertemperature. In this case, the temperature was controlled at 106° C. Thecoated foam was cooled to room temperature after the MOCVD.

A surfactant templated solution was prepared. according to weight ratioof: C₁₆EO₁₀ (polyoxyethylene 10 cetylether):ETOH:TEOS:HCl:Fe(NO₃)₂.9H₂O:H₂O of 17.5:75:40:1:40:100. Thepreparation began with two separate solutions, i.e. an alcohol based andan aqueous solution, prepared according to the weight ratio mentionedabove, then mixed together during the final stirring step.

To prepare the alcohol-based solution, the C₁₆EO₁₀ surfactant was firstdissolved in ethanol under continuous stirring for 1 h at 40° C. on ahot/stir plate. The heat setting on the hot plate was turned off after 1h stirring before TEOS (tetraethylorthosilicate) and 12 M HCl wereconsecutively added into the solution. Between the additions, thesolution was aged and stirred for a 1 h period. The C₁₆EO₁₀(polyoxyethylene 10 cetyl ether):ETOH:TEOS:HCl ratio was 17.5:75:40:1.(Another preparation used 3.16 mL H2O, 8.08 g EtOH, 0.173 g HCl, 4.075 gC₁₆EO₁₀, and 8 mL TEOS. Sometimes, the H₂O was replaced by EtOH, and noHCl was used.

Separately, an aqueous Fe nitrate solution was prepared according to aratio of: Fe(NO₃)₂.9H₂O:H₂O of 40:100. The Fe precursor was firstdissolved in de-ionized water and stirred for at least 1 h. Bothsolutions were mixed together and stirred for another hour prior to dipcoating onto the foam. The excess solution was removed by absorbing ontoa filter paper, then the substrate was calcined at a ratio of 1° C./minfrom room temperature to 450° C., and held isothermally at thattemperature for 1 h under air before cooling to room temperature. Atthis stage, the substrate was ready for the carbon nanotube growth. Thesubstrate was loaded into a 1.25″ OD quartz reactor, heated under 500sccm of N₂ flow from room temperature to 700° C. 500 sccm of ethylenewas introduced into the flow reactor for three-20 min periods, with 5min O₂/N₂ (˜200 ppm O₂) purge between those periods. After the combinedgrowth time of 60 min, the substrate was cooled down from 700° C. to450° C. under N₂. At 450° C., air was introduced to the growth chamberfor 5 min to oxidize the surface of the carbon nanotube before furthercooling to room temperature under N₂. The substrate was then dip coatedwith a colloid alumina solution containing approximately 5 wt % of Al₂O₃which was prepared by mixing a PQ alumina colloidal sol (PQ Corp, AL20DWLot #30-001598, Ashland, Mass.) with a 1:3H₂O/EtOH solvent, followed bydrying at 110° C. before calcining at 350° C. for 3 h in air. Thesubstrate was then dip coated with an aqueous solution containing cobaltand rhenium precursors. Cobalt nitrate hexahydrate and pherrennic acidwere used as the precursors, they were co-dissolved in the 4.883 Mcobalt and rhenium (Co+Re) solution with Co/Re molar ratio of 29.79,dried at 110° C. then calcined at 350C for 3 h. A catalyst (run ID ofMD153 in Table 1 on CNT/FeCrAlY substrate) was prepared containing 0.043g Co—Re/Alumina with 37 wt %/Co4 wt % Re. The weights of FeCrAlY foamsubstrate and carbon nanotubes are 0.3799 g and 0.0675 g, respectively.

Preparation of Fischer-Tropsch Catalysts Without Carbon Nanotubes

Co/Re/alumina catalysts were also prepared on the same FeCrAlY substratewithout a carbon nanotube layer. The FeCrAlY intermetallic foam (80 ppi,85% porosity) with the dimensions of 0.30″×1.4″×0.06″ was first oxidizedat 900° C. in air for 2 h, then was coated with a submicron layer ofAl₂O₃ using MOCVD at 850° C. The MOCVD was carried out using aluminumisopropoxide as the precursor with N₂ carrier gas in an oxidizingenvironment containing 14 vol % of O₂ under 5 Torr at 850° C. Thealuminum isopropoxide precursor was stored in a bubbler where the vaporpressure was controlled by changing the bubbler temperature. In thiscase, the temperature was controlled at 106° C. The coated foam wascooled to room temperature after the MOCVD. The substrate was then dipcoated with a colloid alumina solution containing approximately 5 wt %of Al₂O₃ which was prepared by mixing a PQ alumina colloidal sol (PQCorp, Lot #30-001598, Ashland, Mass.) with a 1:3H₂O/EtOH solvent,followed by drying at 110° C. before calcining at 350° C. for 3 h inair. Upon cooling to room temperature, the substrate was dip coated withan aqueous solution containing cobalt and rhenium precursors. Cobaltnitrate hexahydrate and pherrennic acid precursors were co-dissolved inthe 4.883 M cobalt and rhenium solution with Co/Re ratio of 29.79, driedat 110° C. then calcined at 350C for 3 h. A catalyst (run ID of MD157 inTable 1 on FeCrAlY substrate) was prepared containing 0.0662 gCo/Re/Alumina with 50 wt %/Co5 wt % Re.

Preparation of Fischer-Tropsch Catalysts Without Carbon Nanotubes onOther Metal Substrates

Fischer-Tropsch catalysts were also prepared on various other metal foamsubstrates (Cu, stainless steel, GPM) with the dimensions of0.30″×1.4″×0.06″ without carbon nanotube layer. These metal substrateswere purchased from Porvair (Hendersonville, N.C.) with 80 ppi poredensity and 85% porosity except that GPM (FeCrAlY) has a much higherpore density (>400 ppi with an average pore size of 30 microns) andlower porosity (70%). These catalysts were prepared as follows. First,acidic gamma-alumina support powder (Engelhard) was ground and sieved tobetween 70- and 100-mesh (150 to 220-micron), and calcined (stabilized)at 500° C. for several hours. This powder was then impregnated with asolution containing cobalt nitrate hexahydrate and pherenic acidprecursors, present in desired concentrations to produce a 20-wt %cobalt, 4-wt % Re on alumina catalyst. The precursor solution wasprepared in such a manner as to saturate the pore volume of the aluminasupport without over saturation of the alumina support. This powder wasthen dried in a vacuum oven at 100° C. for at least 4 hours, followed bydrying at 100° C. for at least 12-hours. The powder was then calcined byheating at 350° C. for at least 3 hours. A portion of the powder wasthen combined with distilled water in a water-to-catalyst weight ratioof at least 2.5 to produce a catalyst slurry. This catalyst slurry isthen placed in a container with inert grinding media balls and placed ona rotating device for at least 24 hours. This slurry was then ready tocoat a pre-treated metal foam type support. The metal foam pretreatmentconsisted of cleaning successively in dichloromethane and acetonesolvents in a water bath submersed in a sonication device to agitate thesolvent within the foam. Optionally, the metal surface of the monolithmay then be roughened by etching with acid. If this is desired, themetal foam is submerged in 0.1-molar nitric acid, and placed in asonication device. The metal foam was then rinsed in distilled water anddried at about 100° C. The metal foams, except the Cu foam, were thencoated with a layer of alumina using a metal organic chemical vapordeposition (MOCVD) technique. Cu foam was used after the cleaningwithout the CVD Al₂O₃ coating. The CVD system has a horizontal, hot-wallreactor with three precursor sources. The CVD coatings are performed ata deposition temperature between 600° C.-850° C. and reactor pressure of5 torr. Aluminum iso-propoxide was used as the aluminum precursor. Thisprecursor is stored in a quartz container maintained at 106° C. duringdeposition, which produces a vapor that is carried into the CVD reactorby a flow of nitrogen carrier gas for about 60 minutes. Air was thenused to oxidize the aluminum precursor to alumina. Typical thickness ofthe alumina coatings is about 0.5 μm. This pretreated metal support foamwas then coated with the catalyst slurry by dip coating. The metal foamwas then dried in flowing air or nitrogen at room temperature whilecontinuously rotating the metal foam in such a way as to create auniform coverage of the dried catalyst slurry layer. The metal foam wasthen dried at 90° C. for at least 1-hour, heated slowly to 120° C. overthe course of at least-hour, dried further at 120° C. for at least 2hours, and then heated to 350° C. and calcined for at least 3 hours. Theweights of alumina supported Co—Re powder catalyst on the metal foam arelisted in Table 1.

Catalyst Activity Comparision

The engineered catalysts aforementioned were placed inside the reactionchamber and activated (or reduced) prior to reaction by heating to about350° C. to 400° C. and under flow of a hydrogen-containing stream ofabout 10 to 20% (by mole or volume) hydrogen in an inert carrier gas(such as nitrogen or helium) at a flow rate of at least 20 cc/min(measured at 273 K and 1 atm) for at least 2-hours. The catalyst wasthen allowed to cool to reaction temperatures, about 266° C. Thecatalyst was then exposed to a feed gas comprised of H₂ and CO in adesired ratio of moles of H₂ per mole of CO (2/1). The feed gas flowrate is controllable to allow for precise generation of a desiredcontact time (250 msec). The reaction products were then analyzed toevaluate the conversion of CO and the selectivity towards certainproducts, such as methane. The reaction was conducted at a pressure of16 atmospheres. The results are shown in Table 1. Although carbonnanotube containing catalyst has much lower loading of activecomponents, it has much higher specific activity than other catalystswithout carbon nanotubes while maintaining similar methane selectivity.TABLE 1 Comparison of Fischer-Tropsch Activity with and without CarbonNanotubes*** Temp Weight of Co + CO CH4 specific ID (° C.) SupportCatalyst** Re + Al₂O₃ Conv. selectivity activity* MD 266 Cu 20% Co—4%0.174 46% 30% 1070 144 Re MD 260 SS 20% Co—4% 0.182 50% 32% 1110 145 ReMD 266 GPM 20% Co—4% 0.174 60% 26% 1380 151 Re MD 266 CNT/ 37% Co—4%0.0493 g 42% 27% 2362 153 FeCrAlY Re MD 265 FeCrAlY 50% Co—5%  0.662 g20% 31% 645 157 Re*mmol CO converted per gram of cobalt (total) per hour**The balance is Al₂O₃***All experiments were conducted at 16-atm, H₂/CO = 2, 250-msec contacttime.Listed performance values were gathered after 96-to 120-hrs TOS whencatalyst reached steady state performance.

1. An engineered catalyst comprising: a support material havingthrough-porosity; a layer comprising carbon nanotubes on the supportmaterial; and a surface-exposed catalyst composition.
 2. The catalyst ofclaim 1 wherein the support material has an average pore size, asmeasured by microscopy, of at least 1 micrometer (μm).
 3. The catalystof claim 1 wherein the support material has an average pore size, asmeasured by mercury porisimetry and nitrogen adsorption, of 0.3 to 200μm.
 4. A method of conducting a catalyzed chemical reaction, comprising:passing at least one reactant into a catalyst of claim 2; reacting theat least one reactant to form a product.
 5. A catalyst comprising: asupport; nanotubes disposed over said support; an oxide disposed overthe nanotubes; and a catalyst composition disposed over the oxide. 6.The catalyst of claim 5 wherein the oxide layer comprises a mesoporouslayer.
 7. (canceled)
 8. (canceled)
 9. A method of converting a chemicalreactant, comprising: passing at least one reactant into a reactionchamber; wherein the catalyst of claim 1 is disposed within the reactionchamber; and reacting the at least one reactant in the a reactionchamber to produce at least one product.
 10. The method of claim 9wherein the reaction chamber has an interior with a cross-sectional areaand the engineered catalyst occupies at least 80% of saidcross-sectional area.
 11. The method of claim 10 wherein the reactionchamber is a microchannel and the engineered catalyst comprises amonolith.
 12. A microreactor comprising an array of microchannelswherein each of the microchannels in said array comprises an engineeredcatalyst of claim
 1. 13. The microreactor of claim 12 wherein the arrayof microchannels is in thermal contact with at least one microchannelheat exchanger.
 14. The catalyst of claim 1, wherein the engineeredcatalyst has a volume of at least 5 mm³.
 15. The catalyst of claim 1containing 0.1 to 20 weight % carbon.
 16. The catalyst of claim 1 which,when tested at 265° C., at 16 atm, a H₂/CO ratio of 2, and a 250 mseccontact time, exhibits: a CO conversion of at least 25%, a methaneselectivity of less than 30%; and a specific activity (defined as mmolCO converted per gram of total metal (which does not include metal inoxide support) per hour) of at least
 1500. 17. (canceled)
 18. (canceled)19. A method of forming an engineered catalyst comprising: providing alarge pore support having through porosity; forming carbon nanotubesover the large pore support; and depositing a catalyst compositionprecursor over the carbon nanotubes.
 20. A method of converting achemical reactant, comprising: passing at least one reactant into areaction chamber; wherein the catalyst of claim 5 is disposed within thereaction chamber; and reacting the at least one reactant in the areaction chamber to produce at least one product.
 21. The method ofclaim 20 wherein the at least one reactant is in liquid solution.